Apparatus and method for sequential distraction

ABSTRACT

Apparatus and a method of inserting spinal implants is disclosed in which an intervertebral space is first distracted, a hollow sleeve having teeth at one end is then driven into the vertebrae adjacent that disc space. A drill is then passed through the hollow sleeve removing disc and bone in preparation for receiving the spinal implant which is then inserted through the sleeve.

This application is a continuation of U.S. application Ser. No.09/626,636, filed Jul. 27, 2000; which is a continuation of U.S.application Ser. No. 08/484,927, filed Jun. 7, 1995, now U.S. Pat. No.6,096,038; which is a divisional of U.S. application Ser. No.08/074,781, filed Jun. 10, 1993, now U.S. Pat. No. 5,484,437; all ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to artificial fusion implants to be placedinto the intervertebral space left remaining after the removal of adamaged spinal disc and specifically to the apparatus for and method of,inserting the implants.

2. Description of the Prior Art

For the purpose of achieving long term stability to a segment of injuredspine, a fusion (the joining together of two or more bones via acontinuous bridge of incorporated bone) may be performed. Well-known tothose skilled in such art is the interbody fusion wherein the disc ispartially excised and bone placed within that space previously occupiedby that disc material (between adjacent vertebrae) for the purpose ofrestoring a more normal spatial relationship, and to provide forstability; short term by mechanical support, and long term by thepermanent cross bonding of bone from vertebra to vertebra. For fusion tooccur within the disc space, it is necessary to prepare the vertebrae tobe fused by breaking through, or cutting into, the hardened outsideplates of bone (the endplates) to allow the interposed bone graft tocome into direct contact with the more vascular cancellous (spongy)bone, and to thereby trick the body into attempting to heal thisinduced, but controlled, “fracturing” by both bone production and thehealing of the grafts to both opposed vertebral surfaces such that theybecome one continuous segment of bone.

The purpose of the present invention is to provide an implant, and theapparatus and method of inserting the implant within the intervertebralspace left after the removal of the disc material and permanentlyeliminate all motion at that location. To do so, the device of thepresent invention is space occupying within the disc interspace, rigid,self-stabilizing to resist dislodgement, stabilizing to the adjacentspinal vertebrae to eliminate local motion, and able to intrinsicallyparticipate in a vertebra to vertebra bony fusion so as to assure thepermanency of the result.

At present, following the removal of a damaged disc, either bone ornothing is placed into the remaining space. Placing nothing into thisspace allows the space to collapse which may result in damage to thenerves; or the space may fill with scar tissue and eventually lead to areherniation. The use of bone to fill the space is less than optimal inthat bone obtained from the patient requires additional surgery and isof limited availability in its most useful form, and if obtainedelsewhere, lacks living bone cells, carries a significant risk ofinfection, and is also limited in supply as it is usually obtained fromaccident victims. Furthermore, regardless of the source of the bone, itis only marginal structurally and lacks a means to either stabilizeitself against dislodgement, or to stabilize the adjacent vertebrae.

a. Prior Art Implants

There have been an extensive number of attempts to develop an acceptabledisc prosthesis (an artificial disc). Such devices by design would beused to replace a damaged disc and seek to restore the height of theinterspace and to restore the normal motion of that spinal joint. Nosuch device has been found that is medically acceptable. This group ofprosthetic or artificial disc replacements, seeking to preserve spinalmotion and so are different from the present invention, would include:

U.S. Pat. No. 3,867,728 to STUBSTAD—describing a flexible disc implant.

U.S. Pat. No. 4,349,921 to KUNTZ—describing a flexible disc replacementwith file-like surface projections to discourage device dislocation.

U.S. Pat. No. 4,309,777 to PATIL—describing a motion preserving implantwith spiked outer surfaces to resist dislocation and containing a seriesof springs to urge the vertebrae away from each other.

U.S. Pat. No. 3,875,595 to FRONING—describing a motion preservingbladder-like disc replacement with two opposed stud-like projections toresist dislocation.

U.S. Pat. No. 2,372,622 to FASSIO (France)—describing a motionpreserving implant comprising complimentary opposed convex and concavesurfaces.

In summary, these devices resemble the present invention only in thatthey are placed within the intervertebral space following the removal ofa damaged disc. In that they seek to is preserve spinal motion, they arediametrically different from the present invention which seeks topermanently eliminate all motion at that spinal segment.

A second related area of prior art includes those devices utilized toreplace essentially wholly removed vertebrae. Such removal is generallynecessitated by extensive vertebral fractures, or tumors, and is notassociated with the treatment of disc disease. While the presentinvention is to be placed within the disc space, these other vertebraldevices cannot be placed within the disc space as at least one vertebrahas already been removed such that there no longer remains a “discspace”. Furthermore, these devices are limited in that they seek toperform as temporary structural members mechanically replacing theremoved vertebrae (not a removed disc), and do not intrinsicallyparticipate in supplying osteogenic material to achieve cross vertebraebony fusion. Therefore, unlike the present invention which provides fora source of osteogenesis, use of this group of devices must beaccompanied by a further surgery consisting of a bone fusion procedureutilizing conventional technique. This group consisting of vertebralstruts rather than disc replacements would include the following:

U.S. Pat. No. 4,553,273 to WU—describing a turnbuckle-like vertebralstrut.

U.S. Pat. No. 4,401,112 to REZAIAN—describing a turnbuckle—likevertebral strut with the addition of a long stabilizing staple thatspans the missing vertebral body.

U.S. Pat. No. 4,554,914 to KAPP—describing a large distractible spikethat elongates with a screw mechanism to span the gap left by theremoval of an entire vertebra and to serve as an anchor for acryliccement which is then used to replace the missing bone (vertebrae).

U.S. Pat. No. 4,636,217 to OGILVIE—describing a vertebral strutmechanism that can be implanted after at least one vertebrae has beenremoved and consists of a mechanism for causing the engagement of screwsinto the vertebrae above and the vertebrae below the one removed.

In summary, this second group of devices differs from the presentinvention in that they are vertebral replacements struts, do notintrinsically participate in the bony fusion, can only be inserted inthe limited circumstances where an entire vertebra has been removed fromthe anterior approach, and are not designed for, or intended to be usedfor the treatment of disc disease.

A third area of prior art related to the present invention includes alldevices designed to be applied to one of the surfaces of the spine. Suchdevices include all types of plates, struts, and rods which are attachedby hooks, wires and screws. These devices differ significantly from thepresent invention in that they are not inserted within the disc spaceand furthermore do not intrinsically participate in supplying osteogenicmaterial for the fusion.

Therefore, where permanent spinal immobilization is desired, anadditional surgery, consisting of a spinal fusion performed byconventional means or the use of supplemental methylmethacrylate cementis required. Such devices applied to the spine, but not within the discspace, would include the following:

U.S. Pat. No. 4,604,995 to STEPHENS—describing a “U” shaped metal rodattached to the posterior elements of the spine with wires to stabilizethe spine over a large number of segments.

U.S. Pat. No. 2,677,369 to KNOWLES—describing a metal column device tobe placed posteriorly along the lumbar spine to be held in position byits shape alone and to block pressure across the posterior portions ofthe spinal column by locking the spine in full flexion thereby shiftingthe maximum weight back onto the patient's own disc.

Other devices are simply variations on the use of rods (e.g. Harrington,Luque, Cotrel-Dubosset, Zielke), wires or cables (Dwyer), plates andscrews (Steffee), or struts (Dunn, Knowles).

In summary, none of these devices are designed to be nor can be usedwithin the disc space. Moreover, these devices do not replace a damageddisc, and do not intrinsically participate in the generation of a bonyfusion.

Another area of related prior art to be considered is that of devicesdesigned to be placed within the vertebral interspace following theremoval of a damaged disc, and seeking to eliminate further motion atthat location.

Such a device is contained in U.S. Pat. No. 4,501,269 issued to BAGBYwhich describes an implantable device and limited instrumentation. Themethod employed is as follows: a hole is bored transversely across thejoint and a hollow metal basket of larger diameter than the hole is thenpounded into the hole and then the hollow metal basket is filled withthe bone debris generated by the drilling.

While the present invention (device, instrumentation, and method) mayappear to bear some superficial resemblance to the BAGBY invention, itis minimal, while the differences are many fold and highly significant.These differences include the following:

1. Safety—The present invention provides for a system of completelyguarded instrumentation so that all contiguous vital structures (e.g.large blood vessels, neural structures) are absolutely protected. Theinstrumentation of the present invention also makes overpenetration bythe drill impossible. Such overpenetration in the cervical spine, forexample, would result in the total paralysis or death of the patient. Inthe thoracic spine, the result would be complete paraplegia. In thelumbar spine, the result would be paraplegia or a life-threateningperforation of the aorta, vena cava, or iliac vessels.

The present invention is atraumatically screwed into place while theBAGBY device, in contradistinction, is pounded into position. BAGBYdescribes that its implant is significantly larger in size than the holedrilled and must be pounded in. This is extremely dangerous and thepounding occurs directly over the spinal cord which is precariouslyvulnerable to percussive injury. Furthermore, while it is possible, forexample in the lumbar spine, to insert the present invention away fromthe spinal cord and nerves, the BAGBY device must always be poundeddirectly towards the spinal cord.

Furthermore, since the BAGBY device is pounded into a smooth hole undergreat resistance, and lacking any specific design features to secure it,the device is highly susceptible to forceful ejection which would resultin great danger to the patient and clinical failure. The presentinvention, in contradistinction, is securely screwed into place, andpossesses highly specialized locking threads to make accidentaldislodgement impossible. Because of the proximity of the spinal cord,spinal nerves, and blood vessels, any implant dislodgement as mightoccur with the BAGBY device might have catastrophic consequences.

2. Broad applicability—The BAGBY device can only be inserted from thefront of the vertebral column, however, in contrast, the presentinvention can be utilized in the cervical, thoracic, and lumbar spine,and can be inserted from behind (posteriorly) in the lumbar spine. Thisis of great importance in that the purpose of these devices is in thetreatment of disc disease and probably greater than 99 percent of alllumbar operations for the treatment of disc disease are performed frombehind where the present invention can easily be utilized, but the BAGBYdevice, as per BAGBY'S description, cannot.

3. Disc removal—The BAGBY invention requires the complete removal of thedisc prior to the drilling step, whereas the present inventioneliminates the laborious separate process of disc removal andefficiently removes the disc and prepares the vertebral end plates in asingle step.

4. Time required—The present invention saves time over the BAGBYinvention since time is not wasted laboring to remove the disc prior toinitiating the fusion. Also, with the present invention the procedure isperformed through a system of guarded instrumentation, time is notwasted constantly placing and replacing various soft tissue retractorsthroughout the procedure.

5. Implant stability—Dislodgement of the implant would be a major sourceof device failure (an unsuccessful clinical result), and might result inpatient paralysis or even death. As discussed, the BAGBY device lacksany specific means of achieving stability and since it is pounded inagainst resistance to achieve vertebral distraction, and is susceptibleto forceful dislodgement by the tendency of the two distractedvertebrae, to return to their original positions squeezing out thedevice. The present invention, however, is screwed into place. As thereis no unscrewing force present between the vertebrae, compression alonecannot dislodge the implant. The implant is inherently stable by itsdesign. Furthermore, the threads of the present invention are highlyspecialized in that they are periodically interrupted so that the tailends of each of the tabs so formed are blunted and twisted so as toresist accidental unscrewing. The removal of an implant with such“locking threads” requires the use of a special extractor includedwithin the instrumentation. The stability of the present invention isstill further enhanced, again in contradistinction to the BAGBY device,by the presence of a “bone ingrowth” surface texturing, which bothincreases the friction of the fit and allows for the direct growth ofthe vertebral bone into the casing of the implant itself.

6. Spinal stability—The present invention is not only self-stabilizing,it also provides stability to the adjacent vertebrae in at least threeways that the BAGBY device cannot. First, the BAGBY device is placedtransversely across the joint in the center, leaving both vertebrae freeto rock back and forth over this round barrel shaped axis, much like aboard over a barrel, being used for a seesaw.

Secondly, as the BAGBY device lacks any specific design features toresist sliding, it may actually behave as a third body allowing thetranslation of the vertebrae relative to the device and to each other.

Thirdly, any device can only provide stability if it remains properly,seated. The present invention is inherently stable, and thereforeassures that it will stabilize the adjacent vertebrae, rather than, aswith the BAGBY, the instability of the spine to be treated may cause adislocation of the BAGBY implant, is with further loss of spinalstability.

7. The collapse of the interspace—While both the present invention andthe BAGBY device can be fabricated to withstand the compression forceswithin the interspace, the interspace may nevertheless collapse underthe superincumbent body weight as the implant settles into the vertebralbone. This is related to the load per unit area. Again the presentinvention is superior to the BAGBY device in at least four ways.

First, the present invention offers considerably greater surface area todistribute the load. Secondly, while the BAGBY device is placedcentrally, the present device is placed bilaterally where the bone tendsto be more cortical and much stronger out towards the rim. Thirdly, thepresent invention supports the load achieving an “I” beam effect,whereas the BAGBY implant does not. Fourthly, it is not pressure alonethat causes the collapse of the bone adjacent to the implant, but alsobony erosion that is caused by the motion under pressure of the implantagainst the bone. As discussed in item 6 above, the present inventionalone is highly resistant to such motion, again diminishing thelikelihood of erosion and interspace collapse.

8. Bone ingrowth surface texturing—The present invention has a surfacetreatment of known and conventional technology to induce the growth ofbone from the vertebrae directly into the casing material of the implantitself. The BAGBY device has no similar feature.

9. Fusion mass—The BAGBY invention calls for removing the disc and thendrilling a hole between the adjacent vertebrae. The bony debris sogenerated is then put into the device. The present invention takes acore of pure bone producing marrow from the iliac crest, and then by useof a special press, forcibly injects the implant device with anextremely dense compressed core of that osteogenic material until thematerial itself virtually extrudes from every cell of the implant.

10. The probability of achieving fusion—The fusion rate within the spineis known to be related directly to the amount of exposed vascular bonebed area, the quality and quantity of the fusion mass available, and theextent of the stabilization obtained with all other factors being halfconstant. It would then be anticipated, that the fusion rate would besuperior with the present invention as compared to the RAGBY device,because of optimal implant stability (#5), optimal spinal stability(#6), bone ingrowth surface treatment (#8), superior fusion mass (#9),and the greater exposed vertebral bony surface area (#7).

The last area of prior art possibly related to the present invention andtherefore, to be considered related to “bony ingrowth”, are patents thateither describe methods of producing materials and or materials ordevices to achieve the same. Such patents would include:

-   -   U.S. Pat. No. 4,636,526 (DORMAN), No. 4,634,720 (DORMAN), No.        4,542,539 (ROWE), No. 4,405,319 (COSENTINO), No. 4,439,152        (SMALL), No. 4,168,326 (BROEMER), No. 4,535,485 (ASHMAN), No.        3,987,499 (SCHARBACH), No. 3,605,123 (HAHN), No. 4,655,777        (DUNN), No. 4,645,503 (LIN), No. 4,547,390 (ASHMAN), No.        4,608,052 (VAN KAPEN), No. 4,698,375 (DORMAN), No. 4,661,536        (DORMAN), No. 3,952,334 (BOKROS), No. 3,905,047 (LONG), No.        4,693,721 (DUCHEYNE), No. 4,070,514 (ENTHERLY).

However, while the implant of the present invention would utilize boneingrowth technology, it would do so with conventional technology.

b. Prior Art Instrumentations and Methods

The following is a history of the prior art apparatus and methods ofinserting spinal implants:

In 1956, Ralph Cloward developed a method and instruments which he laterdescribed for preparing the anterior aspect (front) of the cervicalspine, and then fusing it. Cloward surgically removed the disc to befused across and then placed a rigid drill guide with a large foot plateand prongs down over an aligner rod and embedded said prongs into theadjacent vertebrae to maintain the alignment so as to facilitate thereaming out of the bone adjacent the disc spaces. As the large footplate sat against the front of the spine, it also served as a fixedreference point to control the depth of drilling. The reaming left twoopposed resected arcs, one each, from the opposed vertebral surfaces.The tubular drill guide, which was placed only preliminary to thedrilling, was thereafter completely removed. A cylindrical bony dowel,significantly larger in diameter than the hole formed, was then poundedinto the hole already drilled. Cloward's method of instrumentation wasdesigned for, and limited to, use on the anterior aspect and in theregion of the cervical spine only. The hole was midline, which wouldpreclude its use posteriorly where the spinal cord would be in the way.

As the bone graft to be inserted in Cloward's method was necessarilylarger in diameter than the hole drilled, the graft could not beinserted through the drill guide. This mandated the removal of the drillguide and left the graft insertion phase completely unprotected. ThusCloward's method and instrumentation was inappropriate for posteriorapplication.

In addition, the failure to provide continuous protection to thedelicate neural structures from the instruments, as well as the bony andcartilaginous debris generated during the procedure, made Cloward'smethod inappropriate for posterior application. Also, the drill guidedescribed by Cloward could not be placed posteriorly within the spinalcanal, as the foot plate would crush the nerves. Modifying Cloward'sdrill guide by removing the foot plate completely, would still leave theinstrument unworkable as it would then lack stability, and would not becontrollable for depth of seating.

Nevertheless, Wilterberger, (Wilterberger, B. R., Abbott, K. H., “DowelIntervertebral Fusion as Used in Lumbar Disc Surgery,” The Journal ofBone and Joint Surgery, Volume 39A, pg. 234-292, 1957) described theunprotected drilling of a hole from the posterior into the lumbar spinebetween the nerve roots and across the disc space, and then inserting astack of button-like dowels into that space. While Wilterberger hadtaken the Cloward concept of circular drilling and dowel fusion andapplied it to the lumbar spine from a posterior approach, he had notprovided for an improved method, nor had he advanced the instrumentationso as to make that procedure sufficiently safe, and it rapidly fell intodisrepute.

Crock (Crock, H. V., “Anterior Lumbar Interbody Fusion—Indications forits Use and notes on Surgical Technique,” Clinical Orthopedics, Volume165, pg. 157-163, 1981) described his technique and instrumentation forAnterior Interbody Fusion of the lumbar spine, wherein he drilled twolarge holes side by side across the disc space from anterior toposterior essentially unprotected and then pounded in two at leastpartially cylindrical grafts larger than the holes prepared.

A review of the prior art is instructive as to a number of significantdeficiencies in regard to the method and instrumentation for theperformance of Interbody Spinal Fusion utilizing drilling to prepare theendplates.

As the great majority of spinal surgery is performed in the lumbar spineand from posteriorly, a review of the prior art reveals a number ofdeficiencies in regard to the spine in general, and to the posteriorapproach to the lumbar spine specifically. These deficiencies includethe:

1. Failure to protect the surrounding tissues throughout the procedure,specifically, prior to drilling and until after the insertion of thegraft;

2. Failure to contain the debris, bony and cartilaginous, generatedduring the procedure;

3. Failure to optimize the contact of the cylindrical drill hole andbone graft, the mismatch in their diameters resulting in incongruence offit;

4. Failure to determine the optimal drill size prior to drilling;

5. Failure to determine the optimal amount of distraction prior todrilling;

6. Inability to optimize the amount of distraction so as to restore thenormal spatial relationships between adjacent vertebrae;

7. Inability to create sufficient working space within the spinal canal(between the nerve roots and the dural sac) to make the procedure safe;

8. Absent a foot plate on the drill guide, as necessitated by the closetolerances posteriorly, the inability to reliably insure that thedrilling is parallel to the vertebral endplates;

9. The inability to insure equal bone removal from the opposed vertebralsurfaces; and

10. The inability to determine within the spinal canal, the proper sideby side positioning for dual drill holes.

SUMMARY OF THE INVENTION

The present invention comprises a series of artificial implants, thepurpose of which is to participate in, and directly cause bone fusionacross an intervertebral space following the excision of a damaged disc.Such implants are structurally load bearing devices, stronger than bone,capable of withstanding the substantial forces generated within thespinal interspace. The devices of the present invention have a pluralityof macro sized cells and openings, which can be loaded with fusionpromoting materials, such as autogenous bone, for the purpose ofmaterially influencing the adjacent vertebrae to perform a bony bond tothe implants and to each other. The implant casing may be surfacetextured or otherwise treated by any of a number of known technologiesto achieve a “bone ingrowth surface” to further enhance the stability ofthe implant and to expedite the fusion.

The devices of the present invention are configured and designed so asto promote their own stability within the vertebral interspace and toresist being dislodged, and furthermore, to stabilize the adjacentspinal segments.

The apparatus and method of the present invention for preparing thevertebrae for insertion of the implant allows for the rapid and saferemoval of the disc, preparation of the vertebrae, performance of thefusion, and internal stabilization of the spinal segment.

The present invention is a method for Interbody Spinal Fusion utilizingnovel instrumentation, whereby a protective tubular member is placedprior to the drilling part of the procedure and is left in place untilthe graft is fully seated.

In the preferred embodiment two distractors are used to separate twoadjacent vertebrae to a preferred distance. A hollow Outer Sleeve havingteeth at one end is driven into the adjacent vertebrae on one side tohold the vertebrae in position when the distractor is removed, adiameter reducing hollow Inner Sleeve is introduced into the OuterSleeve, a drill having a drill stop is passed through the hollow InnerSleeve to drill a hole to a desired depth, and an implant is inserted inthe hole. The method is repeated on the other side of the disc.

In summary then, the present invention, instrumentation, and method,provides for a single surgery providing for an integrated discectomy,fusion, and interbody internal spinal fixation.

Discussion of the Instrumentation

The apparatus and method of the present invention provide the followingadvantages:

1. The present invention is safer by providing protection of thesurrounding tissues. An Outer Sleeve places all of the delicate softtissue structures, nerves, blood vessels, and organs outside of the pathof the various sharp surgical instruments and the implant. Further, itis an improvement upon hand held retractors in that it occupies theleast possible amount of area, avoids the stretching associated withmanual retraction, provides for the retraction and shielding of thesurrounding tissues in all directions circumferentially andsimultaneously, and it does so exclusively with smooth, curved surfaces.

2. The present invention is safer by providing protection against thedanger of instrument or implant overpenetration.

3. The present invention is safer as the surgical site and wound areprotected from the debris generated during the procedure.

4. The present invention is safer because the method provides forabsolute protection to the soft tissues directly and from indirectinjury by overpenetration. It makes safe the use of powerinstrumentation which is both more effective and efficient.

5. The present invention maintains the vertebrae to be fused rigidthroughout the procedure.

6. The present invention holds the vertebrae to be fused alignedthroughout the procedure.

7. The present invention holds the vertebrae to be fused distractedthroughout the procedure.

8. The present invention assures that all instruments introduced throughthe Outer Sleeve are coaxial and equally centered through the disc spaceand parallel the endplates.

9. The present invention facilitates the implant insertion by counteringthe high compressive forces tending to collapse the interspace, which ifleft unchecked would resist the introduction and advancement of theimplant and make stripping more likely.

10. The present invention extends the range and use of the procedure andsimilarly the interbody spinal implant itself by making the proceduresafe throughout the spine.

11. The present invention increases the ability to use a specificallysized implant.

12. In the present invention the end of all the penetratinginstrumentation is blunt faced.

13. In the present invention all of the instruments have been stopped ata predetermined depth to avoid overpenetration.

14. The design of the Outer Sleeve in the present invention conforms tothe spacial limitations of the specific surgical site.

15. The design and use of a second or Inner Sleeve in the presentinvention allows for the difference in size between the inside diameterof the Outer Sleeve, and the outside diameter of the drill itself. Thisdifference being necessary to accommodate the sum of the distraction tobe produced, and the depth of the circumferential threading present ofthe implant.

16. In the present invention a specially designed drill bit with acentral shaft recess allows for the safe collection of the drillingproducts, which can then be removed without disturbing the Outer Sleeveby removing the drill bit and Inner Sleeve as a single unit.

17. In the present invention a specially designed trephine for removinga core of bone slightly smaller in diameter than the internal diameterof the implant cavity itself, however of a greater length.

18. In the present invention a specially designed press for forcefullycompressing and injecting the long core of autogenous bone into theimplant, such that it extrudes through the implant itself.

19. In the present invention a specially designed driver extractor,which attaches to the implant and allows the implant to be eitherinserted or removed without itself dissociating from the implant, exceptby the deliberate disengagement of the operator.

20. In the present invention predistraction increases the working space.

21. The Distractor in the present invention is self-orienting acting asa directional finder.

22. The Distractor in the present invention is self-centralizing betweenthe opposed vertebral surfaces acting as a centering post for thesubsequent bone removal.

23. In the present invention predistraction assures the equal removal ofbone from the adjacent vertebral surfaces.

24. In the present invention predistraction assures the exact congruencebetween the hole drilled and the device.

25. In the present invention predistraction assures that the drilling isparallel to the vertebral endplates.

26. In the present invention predistraction allows for the determinationof the optimal distraction prior to drilling.

27. In the present invention predistraction allows for the verificationof the correct prosthesis size prior to drilling.

28. In the present invention predistraction facilitates device insertionby relieving the compressive loads across the interspace which wouldresist implantation.

29. In the present invention predistraction decreases the likelihood ofstripping the bone during insertion.

30. In the present invention predistraction provides for the side byside positioning, spacing, and parallelism required prior to theirrevocable event of drilling.

31. In the present invention predistraction provides for the rigidstabilization of the vertebrae opposed to the disc space throughout thesurgical procedure.

32. In the present invention predistraction provides for an implanteasier to insert as the compressive loads of the opposed vertebrae areheld in check so that the device itself need not drive the vertebraeapart to be inserted.

33. In the present invention predistraction allows for the insertion ofa more effective implant as more of the implant can be dedicated to itsintended purpose and be full diameter, whereas without the benefit ofpredistraction and the ability to maintain the same, a significantportion of the forward end of the implant would need to be dedicated tothe purpose of separating the opposing vertebrae.

34. The present invention allows for the use of an implant with asharper thread or surface projections as there is no danger to thesurrounding tissues.

35. The present invention allows for the implant to be fully preloadedas provided to the surgeon, or for the surgeon to load it with thematerial of his choice at the time of surgery.

36. The present invention allows for the loading of a spinal implantoutside of the spinal canal and prior to implantation.

OBJECTS OF THE PRESENT INVENTION

It is an object of the present invention to provide an improved methodof performing a discectomy, a fusion, and an internal stabilization ofthe spine, and specifically, all three of the above simultaneously andas a single procedure.

It is another object of the present invention to provide an improvedmethod of performing a discectomy, a fusion, and an internalstabilization of the spine, which is both quicker and safer than ispossible by previous methods.

It is another object of the present invention to provide an improvedmethod of performing a discectomy, a fusion and an internalstabilization of the spine, to provide for improved surgical spinalimplants.

It is another object of the present invention to provide an improvedmethod of performing a discectomy, a fusion, and an internalstabilization of the spine, which provides for an improved system ofsurgical instrumentation to facilitate the performance of the combineddiscectomy, fusion, and internal spinal stabilization.

It is another object of the present invention to provide an improvedmethod of performing a discectomy, a fusion, and an internalstabilization of the spine procedures.

It is an object of the present invention to provide instrumentation anda method of spinal interbody arthrodesis that is faster, safer, and moreefficacious than prior methods, and can effectively be performed in thecervical, thoracic, and lumbar spine anteriorly, as well as in the lowerlumbar spine posteriorly.

It is a further object of the present invention to provide a means forinserting a spinal implant between adjacent vertebrae while maintainingtheir optimal spacing, positioning, and alignment.

These and other objects of the present invention will be apparent fromreview of the following specification and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of the Long Distractor, of the present inventioninserted into the intervertebral space.

FIG. 2 is a side view of a Convertible Distractor assembly in relationto the spine.

FIG. 3 is a perspective view of a high retention Short Distractor ofFIG. 2.

FIG. 3A is a side view of the high retention Short Distractor of FIG. 2.

FIG. 3B is a side view of an alternative Short Distractor withcircumferential forward facing ratcheting.

FIG. 3C is a top view of the alternative Short Distractor of FIG. 3B.

FIG. 3D is a perspective view of an alternative embodiment of a ShortDistractor.

FIG. 3E is a top view of the alternative distractor of FIG. 3D.

FIG. 3F is a side view of a further alternative rectangularized ShortDistractor with knurled surfaces.

FIG. 4 is a perspective view of a spinal segment (two vertebrae and aninterposed disc) with a Short Distractor in place, with a portion of theupper vertebrae and disc cut away to show the Short Distractor on oneside of the spine and the Long Distractor about to be placedcontralaterally.

FIG. 5 shows a side view of the Outer Sleeve in place over the LongDistractor, and about to receive the Driver Cap in preparation for beingseated.

FIG. 6 shows the Long Distractor, Outer Sleeve, and Driver Cap followingthe proper seating of the Outer Sleeve into the two adjacent vertebrae.

FIG. 7A is a side view of the cervical Outer Sleeve being placed over aLong Distractor which is in place within the disc space anteriorly.

FIG. 7B is a bottom view of the single Outer Sleeve of FIG. 7A.

FIG. 7C is a bottom view of a Dual Outer Sleeve.

FIG. 7D is an enlarged side view of the proximal portion of FIG. 7C.

FIG. 7E is a bottom view of a Dual Driver Cap for driving twodistractors.

FIG. 7F is a side sectional view showing the Dual Outer Sleeve of FIGS.7C and 7D, Distractors and Dual Cap of FIG. 7E seated.

FIG. 8 is a side view of the Outer Sleeve of FIG. 7A centered on theLong Distractor and fully seated on the anterior aspect of the cervicalspine.

FIG. 9 is a perspective view of the Distractor Puller.

FIG. 10 is a cutaway partial side view of the Proximal Puller engagingthe extraction ring of the Long Distractor over the end of the OuterSleeve.

FIG. 10A is a side view of the Puller coupled to the Long Distractorjust prior to its extraction.

FIG. 10B is a posterior view of the proximal Outer Sleeve and a ShortDistractor in place in regard to the vertebrae, disc and nerves.

FIG. 11A is a side sectional view of the Drill and Inner Sleeve withinthe Outer Sleeve and drilling across the intervertebral space andcutting partially cylindrical arcs from the adjacent vertebrae.

FIG. 11B is a sectional side view of preparation of the intervertebralspace by the alternative “Trephine Method” showing the Distractor,Trephine, Inner Sleeve, and Outer Sleeve in place.

FIG. 11C is a sectional side view as in FIG. 11A, but showing the use ofan alternative drilling conformation wherein the extended proximalportion is both distracting and self-centering.

FIG. 11D is a side view of an instrument for removing arcs of bone fromvertebrae following drilling.

FIG. 12 is a perspective view of the surgical Tap.

FIG. 13 is a side view of the Outer Sleeve and the surgical Tap fullythreaded within the interspace.

FIG. 14A is a side view of the bone harvesting Trephine and motoradapter.

FIG. 14B is a perspective view of the implant Bone Loading Device.

FIG. 14C is a perspective view of the Corkscrew bone freeing andextracting instrument.

FIG. 15 is a partial perspective view of the Bone Loading Device inoperation.

FIG. 16 is a perspective view of the Implant Driver about to engage thespinal implant.

FIG. 17 is a side view of the spinal implant being fully seated withinthe intervertebral space by means of the Driver apparatus in placewithin the Outer Sleeve.

FIG. 18 is a side view of the lumbar spine showing the end result of thedevice implantation via the posterior route.

DETAILED DESCRIPTION OF THE DRAWINGS

The following discussion will be in regard to application in the lumbarspine via the posterior approach. In its simplest form, the method ofthe present invention involves the following steps. The patient isplaced on a spinal surgery frame, which allows for the distraction andalignment of the disc space to be fused. A bilateral posterior exposureof the interspace, with or without partial discectomy is then performed.Utilizing distractors the disc space is distracted, and a hollow OuterSleeve is fitted over one of the distractors. The end of the OuterSleeve has teeth for engaging the two adjacent vertebrae. The OuterSleeve is driven into the vertebrae and the distractor is then removed.A hollow Inner Sleeve is then inserted into the Outer Sleeve and astopped Drill is utilized to prepare the opposed vertebral surfaces. TheDrill and the Inner Sleeve are removed as a single unit. The space istapped if so required. The prepared spinal implant is then inserted viathe Outer Sleeve utilizing a stopped inserter. The instruments are thenremoved and the procedure repeated on the contralateral side of thespine.

Step 1a. Prior to surgery, translucent implant templates appropriatelyadjusted for scale are superimposed on AP, lateral, and axial images ofthe interspace to be fused, for the purpose of selecting the optimalimplant size and to determine the desired distraction.

Step 1b. The patient is preferably placed onto a spinal surgery framecapable of inducing both distraction and vertebral alignment.

Step 2. In the preferred embodiment, a standard bilateral (partial)discectomy is performed and any posterior lipping of the vertebralbodies adjacent the interspace is removed. Alternatively, no discmaterial need be removed. In the preferred embodiment, the interspace isexposed by performing bilateral paired semihemilaminotomies andresecting the inner aspects of the facet joints adjacent the spinalcanal while preserving the supra and interspinous ligaments.

Step 3. Beginning on the first side, the dural sac and traversing nerveroot at that level are retracted medially and a Long Distractor theninserted and impacted flush to the posterior vertebral bodies adjacentthat interspace. Long Distractors with working ends of increasingdiameter are then sequentially inserted until the optimal distraction isobtained. This optimal distraction not only restores the normal heightof the interspace, but further achieves a balance wherein the tendencyfor the space to collapse is resisted, which in urging the vertebralbodies apart is being equally resisted by the powerful soft tissuestructures about the spinal segment including the outer casing of thedisc (the annulus fibrosus), various ligaments, capsular structures, aswell as the muscles and other soft-tissue structures. This balanceddistraction not only provides for the spatial restoration of the heightof the interspace, but for considerable stability as the space nowresists further distraction or collapse.

In the preferred embodiment, as the desired distraction is approached,the use of the solid bodied Long Distractors is terminated and adisassemblable Convertible Distractor is placed with tactile and/orradiographic confirmation of ideal distraction. The ConvertibleDistractor is then disassembled such that the Short Distractor portionis left in place and the ultra-low profile head portion being positionedadjacent to the canal floor and safely away from the neural structures.To insure that the Short Distractor remains in place until its removalis desired, various embodiments of the Short Distractor are availablewith varying degrees of resistance to dislodgment. In the preferredembodiment of the procedure, attention is then directed to thecontralateral side of the spine.

Step 4. On the contralateral side of the same interspace the LongDistractor having at its working end the diameter matching the ShortDistractor already in place, is then inserted. If however, due to anasymmetrical collapse of the interspace it is then determined thatgreater distraction is required on the second side to achieve theoptimal stability, then the appropriate Short Distractor would be placedon the second side. Then the Short Distractor would be removed from thefirst side and replaced with a larger Long Distractor so as to bring theinterspace into balance.

In an alternative embodiment, the entire procedure is performed on theone side of the spine utilizing only the Long Distractor prior torepeating the procedure on the contralateral side of the spine. Whilethis method can be performed in accordance with the remaining steps asdescribed in the preferred embodiment, when utilized it is bestperformed using a Trephine which allows the Long Distractor to remain inplace, thereby allowing for interspace distraction otherwise provided inthe first method by the Short Distractor. This alternative method thenrequires the use of a Trephine over the Long Distractor in lieu of areamer and is therefore called the “Trephine Method”, which will bediscussed in detail later.

Step 5. With the Short Distractor in place on the first side of thespine, and the matching Long Distractor in place on the second side ofthe spine, and with the dural sac and traversing nerve root safelyretracted, the Outer Sleeve is placed over the Long Distractor andfirmly impacted to its optimal depth using the Impaction Cap and amallet. The Long Distractor is then removed.

Step 6. An Inner Sleeve is then placed within the Outer Sleeve, and theinterspace is then prepared on that side by utilizing a Drill, Endmill,Reamer, or Trephine to drill, ream, or cut out the bone to be removed toeither side, as well as any remaining interposed discal material. In thepreferred method, utilizing a specially designed Endmill-Drill, it andthe Inner Sleeve are removed as a unit, safely carrying away the boneand disc debris trapped within them from the spinal canal.

Step 7. If required, a thread forming Tap with penetration limitingmeans to control the depth of insertion, is then inserted through theOuter Sleeve.

Step 8. The prepared implant is then inserted utilizing the specializedDriver unit. It should be noted that the implant may be coated with,made of, and/or loaded with substances consistent with bony fusion.However, in the preferred embodiment, the implant is treated with bonepromoting and inducing substances, but is loaded with materials suitablefor participating in a fusion.

While substances both natural and artificial are covered by the presentinvention, the preferred embodiment is in regard to the use of thepatient's own bone by the following method. A hollow Trephine isutilized to harvest a core of bone from the posterior superior aspect ofthe iliac crest adjacent the sacroiliac joint. This core of bone is atits outside diameter, slightly smaller than the inside diameter of thespinal implant to be loaded, but longer than the spinal implant.Utilizing an instrument designed for that purpose, the core of bone isthen injected from within the Trephine into the central cavity of theimplant causing a superabundance of the bone material within the implantsuch that the bone material tends to press out through the openingscommunicating with the outside surface of the implant.

Step 9. Using the Driver Extractor instrument, the prepared implant isthreaded into the prepared interspace. The instrumentation is removedfrom that side of the spine and attention is then redirected to thefirst side of the spine. A small retractor is utilized to move the duralsac and traversing nerve root medially and to protect them and allowingthe direct visualization of the retained Short Distractor unit. Withoutremoving the Short Distractor, it is reassembled to its shaft portion,essentially reconstituting itself into a Long Distractor. With theinserted implant now acting as the distractor on the opposite side, theLong Distractor is utilized to guide the Outer Sleeve down where it isimpacted as described in Step 5.

Steps 6 & 7 are then repeated, completing the procedure at that level.The wound is then irrigated and closed in the routine manner.

REPRESENTATIVE EXAMPLE OF THE PREFERRED METHOD

Through preoperative templating of the patient's anterior posterior,lateral, and axially imaged MRI scan in conjunction with translucentoverlays of the various sized implants, the correct implant diameter andlength are accurately assessed, as well as the correct amount ofdistraction needed to restore the interspace to its premorbid height.The patient is then properly positioned and a bilateral partialdiscectomy performed via paired semihemilaminotomies.

For the purpose of this example, it will be assumed that by preoperativeassessment it was determined that the correct implant would have anexternal diameter of 18 mm and be 26 mm long. Further, the distractionnecessary to restore the height of the interspace would be approximately10 mm. The dural sac and traversing nerve root would then be retractedmedially and protected, while a Long Distractor having an outsidediameter to the barrel portion corresponding to the implant to beinserted, that is 18 mm, and having a diameter at the working end ofperhaps 8 mm, would be inserted. This then being found to be slightlysmaller than optimal by direct observation, a Convertible Distractorhaving in its barrel portion an 18 mm outside diameter, but having inits working portion a 10 mm diameter would then be inserted. Directobservation and/or x-ray then confirming the ideal distraction, theConvertible Distractor would then be disassembled, the barrel and headportion removed, and the Short Distractor portion left deeply embeddedand with its flanged head flat against the canal floor and deep to theneural structures. It would then be safe to allow the dural sac andnerve root to return to their normal positions, which would besuperficial to the flanged portion of the Short Distractor.

Attention would then be directed to the contralateral side. The duralsac and nerve root would then be retracted medially on this second side,and a Long Distractor with an 18 mm diameter barrel portion and a 11 mmworking portion would then be inserted into the interspace and drivenflush to the bone if necessary, such impaction imploding any osteophytesnot already removed, and assuring that the shoulder portion of thebarrel comes to lie flat against the posterior aspects of the adjacentbodies. With the dural sac and nerve root still safely retracted, theOuter Sleeve would then be placed over the Long Distractor and utilizingthe Driver Cap and a mallet, seated to the optimal depth.

In the preferred embodiment, the Long Distractor is then removed and theInner Sleeve is inserted into the Outer Sleeve. Since the purpose of theInner Sleeve is to support the drill and allow for the increased size ofthe implant over the size of the drill, thus making it possible for theinsertion of the implant to occur through the Outer Sleeve, the InnerSleeve therefore measures 18 mm in its outside diameter, and 16.6 mm inits inside diameter. This allows it to fit within the Outer Sleeve, thediameter of which is 18.1 mm and to admit the drill bit which is 16.5 mmin diameter.

Following the drilling procedure, the Drill and Inner Sleeve are removedas a single unit with the trapped interposed cartilaginous and bonydebris. The depth of drill penetration is preset and limited by thefixed rigid column of the Outer Sleeve. In this example, the space willbe prepared to a depth of 28 mm in anticipation of countersinking a 26mm long implant at least 2 mm. If a Tap were to be utilized, it would beinserted at this time and be appropriate to the minor and majordiameters of the implant to be inserted and as with the Drill,controlled for its depth of penetration. The spinal implant would thenbe prepared for implantation by utilizing a Trephine to harvest a coreof posterior iliac bone greater than 30 mm long and approximately 14.5mm in diameter.

Using the Bone Loading Device, this core of bone would be forcefullyinjected into the internal chamber of the spinal implant which wouldthen be capped. Cap end forward, the fully loaded implant would then beattached to the Insertion Driver, down the Outer Sleeve and screwed intoplace with the depth of penetration limited by the Insertion instrument.The Insertion Driver is then unscrewed from the implant and removed fromthe Outer Sleeve. With the dural sac and nerve root retracted andprotected, the Outer Sleeve would then be removed. This would completethe fusion procedure on that side, and then as described, the procedurewould be repeated on the other (first) side of the same interspace.

Alternative Methods

An alternative and extremely useful method is the “Trephine Method”. Itsadvantages include that it may be used in conjunction with the preferredembodiment substituting the use of a hollow, tubular cutter, called aTrephine for the use of the Drill in Step 5 of the preferred embodiment.Additionally, it may be utilized so as to obviate the need for theplacement of the Short Distractor and to allow the procedure to beeffectively performed from start to finish on one side prior toinitiating the procedure on the opposite side, and while neverthelessmaintaining distraction at the site of the bone removal.

The following is a description of the “Trephine Method”. Havingcompleted the exposure of the interspace on at least one side, the duralsac and nerve root are retracted. A Long Distractor differing from theLong Solid Bodied Distractor of the preferred embodiment only in thatthe barrel portion is of a precisely lesser diameter than the spinalimplant. As in the preferred embodiment, the Outer Sleeve has an innerdiameter only slightly greater than the implant to be inserted.Therefore, at this time, a first Inner Sleeve is inserted into the OuterSleeve to make up the difference between the outside diameter of theLong Distractor and the inside diameter of the Outer Sleeve. With theOuter Sleeve and first Inner Sleeve thus assembled, they are placed overthe Long Distractor and the Outer Sleeve is optimally seated using theImpaction Cap. The Cap and first Inner Sleeve are removed, but the LongDistractor and Outer Sleeve are left in place.

With the Long Distractor maintaining optimal distraction and with theOuter Sleeve locking the vertebrae together so as to resist any movementof the vertebrae, a hollow, tubular cutter known as a Trephine is theninserted over the Long Distractor and its barrel portion and within theOuter Sleeve. The Trephine, which is stopped out to the appropriatedepth, can then be utilized to cut equal arcs of bone from the opposedvertebral endplates.

Alternatively, a second Inner Sleeve may be placed within the OuterSleeve prior to placing the Trephine over the Long Distractor and withinthat second sleeve. This second Inner Sleeve would be just greater inits internal diameter than the Long Distractor and just smaller in itsoutside diameter than the inner diameter of the Outer Sleeve. While itwould provide enhanced stability to the Trephine, provision would thenneed to be made in is the way of large flutes passing longitudinally orobliquely along the outer surface of the Distractor to its barrelportion to accommodate the bony and cartilaginous debris generatedduring the cutting procedure.

Following the use of the Trephine to the appropriate depth by either ofthese methods, the Trephine, the Long Distractor, and the second InnerSleeve, if utilized, are all removed. Since the Trephine cuts two arcsof bone but does not ream them out, a shafted instrument with aperpendicular cutting portion at its working end is then insertedparallel to the disc space and then rotated through an arc of motioncutting the bases of the two longitudinally cut arcs, thus freeing themfor removal through the Outer Sleeve. The space may then be tapped ifrequired, and the implant is inserted as per the preferred method. Asalready mentioned, the “Trephine Method” can be used with or without theuse of the Short Distractor on the contralateral side.

Applications of Method in Other Areas of the Spine

The following method is the preferred embodiment for performing anteriorinterbody fusion in the thoracic and lumbar spines. It is alsoappropriate in the cervical spine when the width of the spine anteriorlyis sufficient so that it is possible to place two implants side by sideand such that each intrudes at least several millimeters into thesubstance of the opposed vertebrae and for the length of the implants.

The interspace to be fused is adequately exposed and the soft tissuesand vital structures retracted and protected to either side.Visualization of the broad width of the interspace anteriorly is madepossible by the absence of the neurological structures in relation tothis aspect of the spine. The center line of the anterior aspect of theinterspace is noted and marked. The disc is removed using first a knifeand then curettes and rongeurs as needed. Alternatively, the disc may beleft intact to be removed during the drilling stage of the procedure.However, as per the preferred embodiment of the procedure, havingremoved the great mass of the nucleus and the greater portion of theannulus anteriorly, Long Distractors with progressively increasingdiameters to their working ends are inserted into the interspace at apoint midway between the central marking line and the lateral extent ofthe anterior aspect of the spine as visualized.

The Dual Outer Sleeve with its common Foot Plate and Retention Prongs isthen inserted over either a singly placed Long Distractor and then thesecond Distractor placed, or is placed over both Distractors if alreadyplaced. The Dual Outer Sleeve is then seated firmly against the anterioraspect of the spine. Any spurs which would interfere with the flushseating of the Foot Plate to the anterior aspect of the spine should beremoved prior to inserting the Long Distractors. Once the Outer Sleevehas been optimally seated, one of the Long Distractors is removed and inits place is inserted an Inner Sleeve and drill bit. The drill bit hasas its outside diameter the minor diameter of the implant to beinserted. The Inner Sleeve is essentially equal in thickness to thedifference between the minor and major diameters of the threadedimplant.

A Stopped Drill is then utilized to prepare the opposed vertebralsurfaces and to remove any remaining disc material interposed. Ifrequired, a Stopped Tap may be inserted through the Outer Sleeve andinto the interspace to create a thread form. The properly preparedimplant is then affixed to the Insertion Driver and passed through theOuter Sleeve down into the interspace and inserted until its depth ofpenetration is limited by the stop on the Insertion Driver. With theimplant itself now in a position to act as a distractor, the LongDistractor is then removed from the contralateral side and the procedurerepeated. When both implants are firmly in place, the outer sleeve maythen be removed. The amount of countersinking of the implants may thenbe adjusted under direct vision.

Detailed Description of the Preferred Embodiment Method andInstrumentation

In the preferred embodiment, the disc (D) between adjacent vertebrae (V)is approached via bilateral paired semihemilaminotomies of the adjacentvertebrae. In the preferred embodiment the supraspinous ligament, theinterspinous ligament, the spinous process, portions of the lamina, andmost of the facet joints are preserved. However, while less desirable,these structures may be removed.

In the preferred method, a bilateral partial nuclear discectomy is thenperformed through bilateral openings created through the posterioraspect of the annulus fibrosus. While considered less desirable, discexcision can be delayed and performed simultaneously with the vertebralbone resection during the drilling procedure. Starting on the first sidea dural nerve root retractor is placed such that the dural sac and lowernerve root are retracted medially allowing exposure to one side of aportion of two adjacent vertebral bodies and the interposed discposteriorly.

Referring now to FIG. 1, preferably after removing some portion ofnuclear disc material, a Long Distractor 100 is inserted under directvision into the intervertebral space. The disc penetrating portion 102is essentially cylindrical with a bullet-shaped front end 103 and ashoulder portion 104 where the penetrating portion 102 extends frombarrel 106. The penetrating portion 102 urges the vertebral bodiesapart, facilitating the introduction of the instruments. LongDistractors with sequentially increasing diameter penetrating portions102 are then introduced. As the optimal diameter of penetrating portion102 is achieved, the vertebral bodies to either side are forced intofull congruence and thus become parallel, not only to the penetratingportion 102, but to each other. At this time, any remaining excrescencesof bone of the posterior vertebral bodies adjacent the posterior discwhich have not already been removed are flattened flush to the vertebralbody by the forced impaction, such as by hitting with a hammer flatsurface 109 of crown 110, driving the shoulder 104 against the lippedportions of vertebrae V. Because of the forced opposition of thevertebral endplates to portion 102 with optimal distraction, unit 100will then come to lie absolutely perpendicular to the plane of theposterior bodies and absolutely parallel to the vertebral endplates,allowing optimal alignment for the procedure to be performed.

Penetrating portion 102 is available in various diameters, but all areof a constant length, which is less than the known depth of theinterspace. This combined with the circumferential shoulder 104, whichis too large to fit within the interspace, protects against the dangerof overpenetration. Barrel 106 is of the same diameter as the externaldiameter of the device to be implanted. A recessed portion 108 below thecrown 110 allows for the Long Distractor 100 to be engaged by anextractor unit shown in FIG. 9.

In the preferred embodiment, a Convertible Long Distractor 113 is usedon the first side of the spine. As shown in FIG. 2, the Convertible LongDistractor 113 has a barrel portion 152 separable from the ShortDistractor portion 120. While the initial distraction may be performedwith a solid Long Distractor, as the optimal distraction is approachedthe appropriate Convertible Long Distractor is utilized. The ConvertibleLong Distractor 113 consists of a Short Distractor portion 120 and abarrel 152 having a rectangular projection 134 at one end. The ShortDistractor 120 has an increased diameter head 128, a rectangular slot118 and an internal threaded opening 114. The barrel 152 is hollow andhas an internal shaft 111 terminating in a large diameter hexagonalcrown 115 at one end and a reduced diameter portion 112. The crown has adetent portion 117 in its flat surface. The other end of the shaft 111has a threaded small member 116 that corresponds to threaded opening114. The shaft 111 is prevented from removal from the barrel 152 by setpin 119 passing through the wall of barrel 152 in a convenient manner.The Short Distractor portion 120 is removably attached to the barrelportion 152 via the mating of female rectangular slot 118 and the malemating member 134. The mating held together by utilizing is knob 136 todrive the crown 110 connected to interior shaft 111 having a threadedworking end screw 116 that threads into the female aperture 118 of theShort Distractor portion 120.

Cap 136 has an open socket 138 for fitting around crown 115 and engagesthe reduced diameter hexagonal portion 112 so as to permit the rotationof shaft-111 and threaded male member 116. A detent ball 150 in theinside of the socket 138 engages detent 117 in the crown 115, holdingthem together.

The Short Distractor portion 120 of FIGS. 2, 3, and 3A-3F are designedto provide for high stability when temporarily situated so as to resistinadvertent migration while the surgeon is working on the second side.To that end, the embodiment of the Short Distractor 120 shown in FIGS. 3and 3A has a pair of sharp pegs 126, to embed into the opposingvertebral bodies and forward facing ratchetings 124, that further resistbackward movement. FIGS. 3B and 3C, which show the preferred embodiment,are side and top views of an alternative embodiment of the distractorportion such that the distractor portion to be interposed between thevertebrae is essentially cylindrical, but with circumferential forwardfacing ratchetings 124.

A further alternative embodiment is shown in FIGS. 3D and 3E. This is amore rectangularized design, with forward facing ratchetings, withoutthe sharp prongs 126 of FIG. 3. FIG. 3F is a side view of a furtherembodiment of the Short Distractor 120 shown with knurling, to increasethe interference with the bone surface so as to add stability to theunit and to resist dislodgment. To this end, it is apparent that theworking ends of both the Long and Short Distractors can have a varietyof configurations consistent with their purpose, and that surfaceirregularities as well as the shape of the ends themselves, with orwithout prongs 126, may be utilized to make the Short Distractor 120more resistant to migration.

Once the ideal distraction has been achieved on the first side of thespine, the Convertible Distractor is dissociated, leaving ShortDistractor 120 in place with its rounded external end 128, safely on thecanal floor and deep to the dural sac and nerve root.

As shown in FIG. 4, the surgeon then moves to the other side of thespine at the same disc (D) level, and retracts the dural sac and nerveroot medially, exposing the disc on that side. Long Distractors 100 arethen sequentially inserted into the disc space until the diameter of thedistractor on the second side is at least as big as that on the firstside. If because of some asymmetry of the interspace a larger diameterdistractor is required on the second side to achieve the idealdistraction as compared to the first side, then the second side isfitted with a Short Distractor of the larger diameter, and the surgeonwould then return back to the first side. In that event, the first sideShort Distractor would then be removed and the Long Distractor 100corresponding to the increased diameter of the already placed ShortDistractor 120 would then be inserted. In either event, the operation iscontinued by working on the one side where the Long Distractor is inplace. In this regard, it should be noted, that by the use of such adevice as the Michelson Spinal Surgery Frame, it may be possible toobtain adequate distraction preoperatively such that the surgeon iseither disinclined to use a distractor, or to simply place the correctLong Distractor on the first side and then proceed with the surgicalprocedure on that side before moving to the opposite side. Thesevariations are within the scope of the present invention.

The Long Distractor now serves as both a centering post and an alignmentrod for the hollow Outer Sleeve 140 shown in FIG. 5 which is fitted overthe Long Distractor 100, shown by phantom lines 101 in FIG. 5. The OuterSleeve 140 is metal and has a sharp toothed front end 142 that iscapable of penetrating into and holding fast the two adjacent vertebrae(V). Interrupting the circumferential sharp teeth of 142 are flat,planar areas 152 which serve to resist the further insertion of thesharp teeth into the vertebral bodies. The toothed front end 142 of theOuter Sleeve 140 is a continuation of the tubular shaft 144, which inturn is connected to circumferentially enlarged tubular back end 146having a knurled outer surface 148 for easier manipulation. Analternative embodiment of an Outer Sleeve incorporates an expansile keyhole and slot configuration 154 to either side of shaft 144 along themid-plane of the interspace and parallel to it such that the end 142resists the collapse of the vertebrae (V) to either side of the disc(D), but may nevertheless allow for their further distraction, in theevent the only diameter or the root diameter of the implant is largerthan the hole drilled.

A Driver Cap 160 in-the form of an impaction cap has at its far end aflat, closed-back surface 162 and at its other end a broad, circularopening. The Driver Cap 160 fits over both the Outer Sleeve 140 and theLong Distractor 100. As the Driver Cap 160 is seated, interior surface170 circumferentially engages portion 146 of the Outer Sleeve until theback end 172 engages the internal shoulder 164. As mallet blows areapplied to surface 162, that force is transmitted via the internalshoulder 164 to the Outer Sleeve 140 via its far end 172, seating teeth142 into the vertebral bodies adjacent the disc space D and to the depthof the teeth 142 to the flat portions 152. As the Outer Sleeve 140 isadvanced forward, crown portion 110 of the Long Distractor is allowed toprotrude within the Driver Cap 160 unobstructed until it contacts theinterior flat surface 168. Once crown 110 comes into contact with theflat interior surface 168, then further taps of the mallet will notadvance the Outer Sleeve, any further motion being resisted by the flatshoulder portion 104 of the Long Distractor abutting the hard surfacesof the posterior vertebral bodies. In this way, the Outer Sleeve 140 issafely and assuredly inserted to its optimal depth and rigidly securingthe two opposed vertebrae as shown in FIG. 6.

The Cap 160 is then removed and the Distractor Puller 200 of FIG. 9utilized to remove the Long Distractor 100 from the spine leaving theOuter Sleeve 140 in place. The Distractor Puller 200 has front portion202, a mid portion 204, and a back handle portion 206. At the frontportion 202 of the Distractor Puller 200, a socket 208 is connected toone end of shaft 210 which at its far end is connected to back handleportion 206. The socket 208 has defined within it a cavity 212 that isopen at its front end and funnelized on the interior aspect of itssides. The cavity 212 is constructed so that the head of the DistractorPuller 200 and the partially circumferential flange 218 engages thecircumferential recess 108 of the Distractor 100. The entrance to cavity212 is slightly funnelized, and the leading edges of flange 218 slightlyrounded to facilitate the engagement of recess 108 and head 110 ofDistractor 100, which is further facilitated in that the Driver Cap 160leaves portion 108 of Distractor 100 precisely flush with the backsurface 172 of the Outer Sleeve 140. This provides a large, flat surface172 to precisely guide surface 230 of socket 208, and open portion 212around head 110 while flange 218 engages recess 108. The springloadeddetent ball 228 engages hemispherical depression 112 in the crown 110,shown in FIG. 2. This springloaded detent 228 in engagement withcomplimentary indent 218 protects against the inadvertent dissociationof the Long Distractor from the Puller 200 after the Distractor has beenremoved from within the Outer Sleeve 140 and prior to its removal fromthe wound. Once out of the body, the two instruments are easilydisassociated by freeing the crown portion 110 from cavity 212 by amanual force applied perpendicular to their relative long axes at thislocation.

A cylindrical and free removable weight 216 is fitted around shaft 210between the front portion 202 and the rear handle portion 206. Gently,but repeatedly sliding the weight 216 along shaft 210 and drivenrearwardly against flat surface 228, transmits a rearward vector toproximal end 202 and thereby to the Long Distractor 100 to which it isengaged.

Paired extended handle 224 and 226, allow the surgeon to resist anyexcessive rearward motion as the instrument is used to liberate the LongDistractor 100. Paired handles 224 and 226 are also useful in that theyallow a rotational directing of portion 208, via the shaft 210. Thisallows the surgeon to control and manipulate rotationally theorientation of the opening of cavity 212 to facilitate its application,to the head 110 of the distractor 100.

The Distractor Puller 200 is a significant improvement over thealternatives of striking a remover instrument with an independent hammerover the exposed surgical wound, or manually extracting the distractorby forcefully pulling. The use of a free hammer over the open wound isdangerous because the neural structures can be impacted on the backswing which is made even more likely by the effects of gravity on themallet head. Manual extraction by pulling is dangerous because of thesignificant interference fit of portion 102 within the spine such thatsignificant force would be required to remove the Distractor 100, and ifforce were not coaxial then the Outer Sleeve might be dislodged ormisaligned. Further, once the flat portion 102 became free of theinterspace, all resistance to withdrawal would be lost and in the faceof the considerable force necessary to free it, the Distractor 100 mighteasily become projectile imparting injury to the patient and/or thesurgeon.

Once the Long Distractor 100 has been fully removed from the OuterSleeve 140, the toothed end 142 of the Outer Sleeve 140, working inconjunction with the Short Distractor 120 on the contralateral siderigidly maintains the relative position of the adjacent vertebrae V.Further, since the remainder of the procedure on that side of the spineoccurs entirely through the protective Outer Sleeve 140, and as thenerves and dural sac are external to that Outer Sleeve and superficialto the toothed end 142 of the Outer Sleeve 140, which is firmly embeddedinto the adjacent vertebrae V, the Outer Sleeve 140 serves to insure thesafety of these delicate neural structures. Further, since the OuterSleeve 140 is of a fixed length and rigid, its flat rearward surface 172may be used as a stop to the advancement of all instruments placedthrough the Outer Sleeve 140, thus protecting against accidentaloverpenetration. Further, the Outer Sleeve 140 assures that the furtherprocedure to be performed will occur coaxial to the disc space D andfurther, be symmetrical in regard to each of the opposed vertebralsurfaces.

FIG. 10B is a posterior view of the spine at this stage of theprocedure, showing a Short Distractor 120 in place on one side of thespine and the bottom portion of Outer Sleeve 140 in place on theopposite side of the spine.

Referring to FIG. 11A, an Inner Sleeve 242 is inserted from the rearwithin the Outer Sleeve 140. This Inner Sleeve has a collar portion 244of a known thickness which seats against the top edge surface 172 ofOuter Sleeve 140. The cylindrical barrel portion of Inner Sleeve 242comes to approximate the posterior aspect of the vertebral bodiesinterior the Outer Sleeve when fully seated. A Drill 240, having a knownselected length is then introduced through the rearward aperture of theInner Sleeve 242 and utilized to ream out the arcs of bone which itengages from the opposed vertebral endplates as well as any discalmaterial within its path down to its predetermined and limited depth.The Drill 240, has a narrow engagement portion 246, which allows it tobe affixed to a drill mechanism which may be either a manual or a powerunit. A circumferential collar 248 of an increased diameter serves tolimit the depth of penetration of the drill 240 and may be fixed, orlockably adjustable.

Not shown here, but well known to those skilled in the art, are variousmechanisms to lockably adjust such instruments as drills. Suchmechanisms include, but are not limited to, the use of collets, threadedshafts with lock nuts, and flanges engaging grooves forced therein byeither a cap pulled over the flanges or screwed down upon them.

In the preferred embodiment, the forward cutting edge 252 of Drill 240is a modification of a large fluted drill design such that the endresembles an end cutting mill which may contain any workable number ofcutting surfaces, but preferably four or more, and such cutting surfacesbeing relatively shallow such that the advancement of the instrumentoccurs more slowly. The outside diameter of the Drill 240 corresponds tothe minor diameter of the threaded spinal implant. The Inner Sleeve 242has an inner diameter slightly greater than that dimension and its outerdiameter is slightly smaller than the inside diameter of the OuterSleeve 140 which has the same outer diameter as the major diameter ofthe threaded implant.

The drill shaft of drill 240 comprises an upper portion 243, a centralrecessed portion 256 of a smaller diameter and a lower cutting drillportion 250. The upper portion 243 and lower portion 256 of the drill240 have the same outside diameter.

The Inner Sleeve 242 serves many functions. First, it provides a moreintimate drill guide for drill 240 in the event a smaller diameter holeis to be drilled than that of the inside diameter of the Outer Sleeve140. Second, since it now guides the Drill, it allows for the OuterSleeve 140 to have an internal diameter large enough to admit thethreaded spinal implant, which is indeed considerably larger in diameterthan the Drill 240 itself.

If a larger Outer Sleeve 140 were utilized absent the Inner Sleeve 242,then the Drill 240 would be free to wander within the confines of thatgreater space and would not reliably make parallel cuts removing equalportions of bone from the adjacent vertebrae V. Further, the boneremoval not only needs to be equal, but must be correctly oriented inthree dimensions. That is, the path of the Drill 240 must be equallycentered within the disc space, parallel the endplates, and parallel tothe sagittal axis dissecting the interspace.

A further purpose of the Inner Sleeve 242 is that it may be removedsimultaneously with the Drill 240, thereby trapping the debris, bothcartilaginous and bony generated during the drilling procedure, whichare guided rearward by the large flutes 251 of Drill portion 250, wherethey are collected around recessed portion 256 between the recessedportion 256 and the inner wall of the Inner Sleeve 242 are therecontained therein. Thus, by removing the Drill 240 in conjunction withthe Inner Sleeve 242, all of the debris generated by the reamingprocedure is safely removed from the spinal canal and wound area.

Further, if the disc tissue in the area to be reamed has been removedpreviously, as per the preferred method, then the patient's own bone ofgood quality and useful within the operation will then be containedbetween the Inner Sleeve 242 and the shaft portion 256. Once away fromthe surgical wound, this material may be used to load the spinal implantor placed deep within the interspace to participate in the fusion.

The method of actually producing the surgical hole within the spine isvariable. As shown in FIG. 1C, in an alternative embodiment Drill end250 has a forward projecting nipple 260, which itself is bullet-shapedin its leading aspect so as to ease its entrance into the disc space andto urge the vertebrae apart. Nipple 260 is distracting, stabilizing asit resists any tendency of the vertebrae to move together, isself-centering to the Drill portion 250 when working in conjunction withSleeves 140 and 242, and virtually assures the symmetrical resection ofbone from the opposed vertebral surfaces.

The alternative “Trephine Method” referred to earlier in thisapplication, is shown in FIG. 11B. In this alternative, a LongDistractor 100 is left in place after the Outer Sleeve 140 is seated.The Long Distractor 100 in this case differs from the Long Distractor ofthe preferred embodiment in that its outside diameter of the barrel 106is of a smaller diameter than in the prior version. This is madenecessary because regardless of the method, the hole to be formedcorresponds to the minor diameter of the spinal implant. Trephine 270, ahollow, tubular member with sharp cutting teeth 251 at its proximal end,has a wall thickness and since the outside diameter of that trephine 270must correspond to the root diameter of the implant, then the wallthickness of the trephine 270 must be allowed for by a correspondingreduction in the diameter of the Long Distractor 100.

A further modification of the Long Distractor 100 to the “TrephineMethod” would use longitudinal grooves (not shown) along the barrelsurface 106 for the purpose of transmitting any debris generated duringthe cutting procedure, rearward. Since the cutting element is bothcentered and aligned by the Long Distractor, the use of the Inner Sleeve242 is not mandatory, but may once again be useful in controlling thepath of the debris. To that end, little debris is generated in the“Trephine Method” as the bony arcs are not so much being reamed out andremoved as they are simply being cut into the bone where these arcs ofbone are left connected at their far ends. Thus, when the TrephiningMethod has been completed and the Trephine 270 and Inner Sleeve 242removed, unlike in the preferred embodiment where the hole is drilledout, it remains necessary to remove both the two arcs of bone, and anyinterposed material. Nevertheless, this is very easily performed byvarious means, one of which is depicted in FIG. 11D.

Instrument 272 consisting of a shaft 276 attached off center to thelower surface 273 handle 274. The shaft 274 terminates in a cutting arm278. The instrument 272 is inserted through Outer Sleeve 140 where thelower surface 273 of handle 274 abuts the top 172 of the Outer Sleeve140, both stopping downward motion of instrument 272 and preciselyplacing the perpendicularly cutting arm 278 of instrument 272 so that ashandle portion 274 is rotated, the cutting arm 278 is also rotated,cutting the arcs of bone and liberating them from their lastattachments. These portions of bone are then removed utilizing thisinstrument or a long forceps, and then placed within the implants orotherwise used to participate in the fusion.

While in the preferred embodiment of the present invention the spinalimplant I, is essentially self-tapping, if the bone is unusually hard itmay be desirable to form the thread pattern within the interspace priorto the insertion of the implant I. To that end, as shown in FIG. 12, Tap280 has a thread-cutting portion 282 connected by a shaft 286 to ahandle portion 292, which has been designed to give mechanical advantageto the rotation of the instrument for the purpose of cutting threads.The lower portion of handle 290 has a forward facing flat surface 288too large to fit through the opening of Outer Sleeve 140 which thussafely limits the depth of penetration of the cutting element 282. Thistap 280 is further made safe by blunt end 294 which will engage theuncut portions of the vertebral bone just prior to the engagement ofshoulder 288 against surface 172. This feature allows the surgeon toappreciate a less harsh resistance as the blunt nose 294 encounters theremaining unresected bone for the drill hole and prior to the suddenincrease in resistance caused by the seating of shoulder 288 against topedge 172, which first resistance serves as a warning to the surgeon todiscontinue the tapping procedure. Thus, the surgeon has both visual (asshoulder 288 approaches top edge 172) and tactile warnings to avoidstripping the thread form. Tap end 282 is highly specialized for itsspecific purpose. Rearward to the specialized blunt tip 294 is atruncated bullet-shaped area 298 which ramps up to the constant diameterintermediate the cutting ridges 296. Ramp portion 298 urges the opposedvertebral bodies apart, which motion is resisted by Outer Sleeve 140,thus progressively driving the sharp leading edges of thread forms 296into the vertebral bodies. The periodic longitudinal grooves 284interrupting the thread forms, which may number 1 to 8, but preferably4, function to accumulate the bony material which is removed during thethread cutting process. In that regard, in the ideal embodiment, thethread cutting form is designed to compress the bone to be formed ratherthan to trough through it. Further, while both the major and minordiameters of the Tap 280 may be varied, in the preferred embodiment, theminor diameter corresponds to the minor diameter of the implant I, butthe major diameter is slightly less than the major diameter of theimplant.

With Tap 280 now removed, and Sleeve 140 still in place, the surgicalsite is now fully prepared to receive the spinal implant I. In thepreferred embodiment of the spinal implant, the implant has beenenhanced by the use of, application to, and filling with fusionpromoting, enhancing, and participating substances and factors. Thus,the implant may be fully prepared for insertion as provided to theoperating surgeon. However, at the present time, human bone is mostcommonly used as the graft material of choice, with the patient's ownbone being considered the best source.

FIG. 14 a shows a trephine 300 with an exceedingly sharp front cuttingedge 302 for quickly and cleanly coring into the patient's posterioriliac crest, or any other bony tissue, and for the purpose of producinga core of bone then contained within the hollow 304 of the trephine 300.Trephine 300 has a rear portion 306 with a pair of diametrically opposedslots 310, and disposed clockwise from their longitudinally orientedrearward facing openings so as to engage diametrically and opposingmembers 312 of Drive unit 308, by which trephine 300 may be attached toeither a hand or power drill. It can be appreciated that engagementmechanism 312 is stable during the clockwise cutting procedure, and yetallows for the rapid disconnection of the two components once thecutting is completed.

Because of the high interference between the graft and the inner wall ofhollow portion 304, and the relative weakness of the cancellous bonebeing harvested, it is possible to remove the Trephine 300 while stilldrilling, and to have it extract the core of bone with it. However, inthe highly unlikely event that the core of bone would remain fixed atits base, then with the drive mechanism 308 removed, a corkscrew 408shown in FIG. 14C is introduced though the central opening of rearportion 306 and threaded down and through the core of bone within 304and to the depth of teeth 302. The tip 318 of the corkscrew 408, whichextends substantially on line with the outer envelope of the corkscrew,then cuts radially through the base of the bone core. As the handleportion 314 of the corkscrew 408 abuts the flat, rearward surface ofportion 306 and it can no longer advance. As corkscrew 408 is continuedto be turned further, it will cause the core of bone to be pulledrearward, as in removing a cork from a wine bottle. Trephine 300 has abarrel portion 304 continuous with sharp toothed portion 302 having aninner diameter just less than the inner diameter of the spinal implant Ito be loaded.

The Trephine 300 with its core of harvested bone is then placed as shownin FIG. 14B, through opening 340 of Implant Bone Loading device 320,where the barrel portion 304 then passes through and is stopped bycircular flange 344. The plunger shaft 326 of instrument 320 is thenprepared for attachment by rotating knob 332 counterclockwise such thatthe plunger 372 is pulled via the long threaded shaft portion 328 backto the base of collar 330 at its proximal end. In this position, knob332 is considerably extended rearward from collar 330. With plungershaft 326 in this position, the plunger head 372 is inserted into thecentral hollow of portion 306 of Trephine 300 as the proximalcylindrical portion of collar 330 then follows it, such that the plunger372 then occupies the rearward portion of barrel 304 and the proximalcylindrical portion of collar 330 occupies the central hollow of portion306. A pair of diametrically opposed radially projecting arms 346 oncollar 330 are then advanced longitudinally into diametrically opposedpaired L slots 340 and then rotated clockwise to complete this assembly.

At the other end of instrument 320, a spinal implant I is engagedthrough its female rectangular slot 364 by a rectangular protruding barextending from rearward facing surface of end plug 324, (not shown) andsecured there by knob 334 which extends as a rod through a centralaperture within end plug 324 to extend at the far end as a small boltwhich threads to a female aperture centered within the female slot 364of the spinal implant. With the spinal implant I secured to end plug 324and the opposite end of the implant I presenting as a hollow, tubularopening, end plug 324 is advanced into device 320 where it is secured byrotationally engaging diametrically opposed L-shaped slots 321. Withdevice 320 fully assembled, end 302 of trephine 300 lies coaxial andopposed to the open end of implant I.

As shown in FIG. 15, as knob 332 is then rotated clockwise, the plunger372 proximal the threaded shaft 328 is then forcibly, but controllablydriven forward down the barrel 304 ejecting the bone graft directly intothe spinal implant I. As the bone graft is greater in length than theinterior of the spinal implant, with further compression the bone isforced into the radially disposed apertures through the wall of thedevice communicating from the central cavity to the exterior.

End plug 324 is then removed from apparatus 320. Using end plug 324 as ahandle, end cap 374 shown in FIG. 16 is secured to the open end of thespinal implant I. The implant is then disassociated from end plug 324 byrotating knob 334 counterclockwise.

FIG. 16 shows an Implant Driver instrument which may be used to eitherinsert or to remove said implant I. Driver 350 has at its far end 362, arectangular protrusion 398, which protrusion intimately engages thecomplimentary rectangular slot 364 of implant I. Protruding from slot398 of end 362 is threaded portion 353, which extends as a rod throughhollow shaft 358 and hollow hand barrel 360 to knob 354 where it can berotationally controlled. Threaded portion 353 screws into a femaleaperture central slot 364, urging 353 into 364, and binding themtogether such that instrument 350 can be rotated via paired anddiametrically opposed extending arms 366 and in either direction whilemaintaining contact with the implant.

Affixed to the Driver 350, the implant is then introduced through theOuter Sleeve 140 and screwed into the interspace opposed between the twoprepared vertebrae V until such time as the leading edge of the ImplantCap 374 reaches the depth of the prepared hole at which time its forwardmotion is impeded by the bone lying before it which had not been drilledout. This allows for a progressive feel to the surgeon as the implant isscrewed home.

As described previously, with the use of the Tap 280, this terminalresistance to further seating provides significant tactile feedback tothe surgeon. Again, as with the Tap 280, visual monitoring of the depthof insertion of the implant is provided to the surgeon by observing theprogressive approximation of the forward surface 370, of barrel portion360, as it approaches the rearward facing surface 172 of Outer-Sleeve140. Nevertheless, a final safety mechanism, when the full depth ofinsertion has been achieved, surface 370 of instrument 350 will abutsurface 172 of the Outer Sleeve 140, prohibiting any furtherinstallation of the spinal implant.

Once the implant has been fully installed, the Driver 350 is dissociatedfrom the implant by turning knob 354 in a counterclockwise direction.The Driver 350 is then withdrawn from the outer sheath, then the OuterSleeve 140 is removed. This leaves the implant fully installed and insetto the determined depth as shown in FIG. 18.

Attention is then redirected to the other, or first, side of the spine.A dural nerve root retractor is used to retract the neural structuresmedially, bringing into full view the head 128 of the Short Distractor120, lying flush on the canal floor. Utilizing apparatus 152, extendedscrew portion 116 is inserted into the female threaded portion 114 ofthe Short Distractor 120 as the extended rectangular portion 134 ofapparatus 152 is engaged to the female rectangular portion 118 of theShort Distractor 120. Then turning rearward facing portions 108 and 110,utilizing the knob 136 of FIG. 2, the Long Distractor configuration isrestored.

With the dural sac and nerve roots still retracted and protected, theOuter Sleeve 140 is slipped over the reconstituted Long Distractor andseated using the Driver Cap 162. The entire sequence of events asdescribed for the implantation of the spinal implant I as alreadyplaced, is then repeated such that both spinal implants come to lie sideby side within the interspace. Though not necessary, circlage or otherinternal fixation of the levels to be fused may additionally beperformed, and then the wound is closed in the routine manner.

Brief Discussion with Reference to the Drawings of the Preferred Methodand Instrumentation for Anterior Interbody Fusion IncorporatingIntercorporeal Predistraction and Utilizing a Guarded Sleeve System isDisclosed

Because of the absence of the spinal cord and nerve roots, it isgenerally possible to visualize in one instance the entire width of thedisc space from side to side throughout the cervical, thoracic, orlumbar spine. In the preferred embodiment of the anterior interbodyfusion, implants are placed side by side from anterior to posteriorparallel to the interspace and extending through into the adjacentvertebral bodies. Where the transverse width of the disc space isinsufficient to allow for the use of two implants, each of which wouldbe large enough to protrude to the required depth into the adjacentvertebrae, then a singular and significantly larger implant may beplaced centrally. With this in mind, and in light of the very detaileddescription of the technique and instrumentation already provided inregard to the method of posterior lumbar interbody fusion, a briefdiscussion of anterior spinal interbody fusion with dual implantinstallation will suffice, and the method for installation of a large,singular midline graft will become obvious.

The interspace to be fused is exposed anteriorly. The soft tissues arewithdrawn and protected to either side, and if necessary, above andbelow as well. It is then possible to visualize the entire width of thevertebrae anteriorly adjacent that interspace. As discussed above, thesurgeon has already templated the appropriate patient radiographs todetermine the requisite distraction and optimal implant size. In thepreferred method, the surgeon then broadly excises the great bulk of thenuclear disc portion. (Alternatively, the disc can be left to be removedvia the drill later.) The surgeon then notes and marks a point midwayfrom side to side anteriorly. He then inserts Long Distractor 100centering it on a point midway between the point just noted and thelateral extent of the intervertebral space visualized anteriorly. Theouter barrel portion 106 of the Distractor 100 utilized, will correspondto the outside diameter of the implants to be installed. The Distractortips 102 inserted are sequentially larger in diameter until the optimaldistraction is achieved. This optimal distraction, although suggested bythe initial templating, may be visually and tactilely confirmed asperformed. When the optimal distraction is achieved, the vertebralendplates will come into full congruence and parallel to the forwardshaft portion 102 of the Distractor 100, causing an alteration in thealignment of the vertebrae and a significant increase in theinterference fit and pressurization at the tip, such that the instrumentbecomes exceedingly stable.

There is a sensation imparted to the surgeon of the tissues having movedthrough their elastic range to the point where the two adjacentvertebrae V begin to feel and move as if a single solid. These changesare easily appreciated visually as the vertebrae realign to becomecongruent to tip 102, and can also easily be appreciated via lateralRoentgenography. However, should the surgeon fail to appreciate thatoptimal distraction has been achieved and attempt to further distractthe interspace, he would find that extremely difficult to do because ofthe increased resistance as the tissues are moved beyond their range ofelastic deformation. Further, there would be no elasticity left to allowthe vertebrae to move further apart and the sensation to the surgeonshould he attempt to gently tap the oversized Distractor forward with amallet, would be one of great brittleness.

Returning now to the procedure, when the correct intercorporealDistractor 100 producing the ideal interspace distraction having itsbarrel portion 106 corresponding to the implant to be installed has beeninserted, then its exact duplicate is inserted anteriorly equidistant tothe other side of the spine. As the barrel portion 106 of LongDistractor 100 is exactly of the same major diameter as the spinalimplant I looking coaxially on end, the surgeon can then asses theanticipated side by side relationship of the dual implants whenimplanted.

As shown in FIGS. 7C and 7D, a Dual Outer Sleeve 340 consisting of apair of hollow tubes is then introduced over the side by side LongDistractors protruding anteriorly from the spine. The Dual Outer Sleeve340 is comprised of two hollow tubular members identical in sizedisplaced from each other ideally the sum of the difference between theminor and major diameters of both implants combined, but not less thanthat difference for one implant, as it is possible to have the threadsof one implant nest interposed to the threads of the other, such thatthey both occupy a common area between them. However, while thepreferred embodiment is slightly greater than two times the differencebetween the major and minor diameters of the implant (the sum of both)the distance may be considerably greater. Whereas in the preferredembodiment extending tubular portions 348 of instrument 340 areparallel, when the area between them 350, is sufficiently great, theseelements may be inclined or declined relative to each other such thatthey either converge or diverge at their proximal ends. Paired tubularstructures 348, may be bridged in part or wholly throughout theirlength, but are rigidly fixed by Foot Plate 344. In its preferredembodiment, a top view shows the Foot Plate to be essentiallyrectangular, but without sharp corners.

Other shapes can be utilized. In side view 7D it can be appreciated,that Foot Plate 344 is contoured so as to approximate the shape of thevertebrae anteriorly. Extending forward from Foot Plate 344 are multiplesharp prongs 342 sufficiently long to affix them to the vertebrae. Theprongs 342 are limited in length so as to not penetrate too farposteriorly and number from 2 to 10, but preferably 6. As the Dual OuterSleeve 340 is driven forward utilizing Dual Driver Cap 420, of FIG. 7E,engaging the rearward end 352, the prongs 342 extending from Foot Plate344 are embedded into the opposed vertebral bodies until their forwardmotion is inhibited by the curved Foot Plate 344 becoming congruent toand being stopped by, the anterior aspect of the vertebral bodies.

As already taught in FIG. 5, the Dual Driver Cap 420 is of the samedesign as Single Driver Cap 160, in that there is a recess 354 as per168, allowing the Outer Sleeve to be fully seated without impeding therearward projection of the Long Distractor unit. However, unlike in Cap160, area 354 is more relieved as it is unnecessary for the Dual Cap 420to contact the Long Distractor through portion 110 to inhibit itsforward motion, as the Foot Plate 344 functions to that effect. Further,the Dual Cap 420 for the Dual Outer Sleeve 340 is correspondingly dualitself and engages the rearward facing dual tubular portion 352. Oncethe Dual Outer Sleeve has been fully seated, the vertebrae adjacent theinterspace to be fused are rigidly held via Foot Plate 344 and theprongs 342. Thus, it is possible to remove either one, or if desired,both of the Long Distractor rods utilizing Long Distractor puller 200,as per the method already described. It is then the surgeon's choice towork on one or both sides of the spine. As per previous discussion, thesurgeon may drill the interspace utilizing the Inner Sleeve 242 or leavethe Long Distractors in place as per the “Trephine Method”.

Tapping, if necessary, and the insertion of the implants then occursthrough the protective Outer Sleeve 340. Once the implants have beenfully inserted, the Outer Sleeve is removed.

Having utilized the Drill method, or “Trephine Method”, with or withoutan Inner Sleeve to prepare the fusion site, it is the preferredembodiment to leave the Outer Sleeve 340 in place as it provides for theideal placement and alignment of the Tap 280 and implant I.

It is anticipated that the surgeon wishing to work deep within theinterspace, or preferring the ability to directly visualize the tapbeing used, or the implant being inserted, may choose to remove theOuter Sleeve after the insertion of the first prosthesis to maintainstability, or prior to that, which while not the preferred embodiments,are nevertheless within the scope of the present invention.

Alternative Methods to the Preferred Embodiment for Method of AnteriorInterbody Fusion

As previously described for the posterior lumbar spine, alternatively,one can employ the “Trephine Method” as has been described in detail.

As a further alternative, it should be noted that the key element in theanterior method is the use of the predistraction principle, where suchdistraction is maintained by the Outer Sleeve with or without the LongDistractor. Therefore, once the preparation of the interspace has beencompleted, while not the preferred embodiment, it is nevertheless withinthe scope of this invention that one could remove the Outer Sleeve asthere are no neural structures requiring protection, and insert theimplants directly rather than through the Outer Sleeve.

As yet a further alternative of this method, where the height of thedistracted interspace is such that the diameter of the implant requiredto span that height and to embed with sufficient depth into the opposedvertebral bodies is such that it is not possible to place two suchimplants side by side, then only a single implant which may be ofsignificantly increased diameter, is used and placed centrally withinthe interspace rather than to either side. The placement of a singularcentral graft via the present invention method and instrumentation is inkeeping with the methods already described and can be performed usingeither a drill or the “Trephine Method”.

Referring to FIGS. 16-18, a cylindrical embodiment of the spinal implantI of the present invention is shown. In FIG. 16 the implant I is shownattached to the insertion device 350. In FIGS. 17 and 18 the implant Iis shown installed in the disc space D, between the adjacent vertebrae.

The cylindrical implant I comprises a hollow tubular member which in thepreferred embodiment is made of an ASTM surgically implantable material,preferably Titanium. The cylindrical implant I is closed at one end andopen at the other end covered by a cap 394. The cylindrical implant Ihas a series of macro-sized openings 390 through the side walls of thecylindrical implant I. A series of external threads 392 are formed onthe circumference of the cylindrical implant I. Any variety of threadsmay be used on the implant. The cap 374 has a hexagonal is opening 394for tightening the cap 374.

While the present invention has been described in association with theimplant of a threaded spinal implant, it is recognized that other formsof implants may be used with the present method. For example, dowels,made from bone or artificial materials, knurled or irregularly shapedcylinders or spheres, or any other shaped implants that can beintroduced through the outer sleeve may be used. Being able to performthe procedure through the outer sleeve permits the procedure to beperformed safely and quickly, and more accurately.

1. A system for spacing apart an upper vertebral body and a lower vertebral body adjacent a disc space in a human spine, the system comprising: a first distractor having a proximal end, a distal end, and a first thickness adapted to separate the upper vertebral body from the lower vertebral body along the first thickness, the first distractor being configured to contact at least a portion of the endplates of the upper vertebral body and the lower vertebral body; a second distractor having a proximal end, a distal end, and a second thickness that is greater than the first thickness, the second distractor adapted to separate the upper vertebral body from the lower vertebral body along the second thickness, the second distractor being configured to contact at least a portion of the endplates of the upper vertebral body and the lower vertebral body; and a distraction maintenance device having an upper surface, a lower surface, an insertion end, and a trailing end, the distraction maintenance device having a height adapted to maintain a space between the upper vertebral body and the lower vertebral body.
 2. The system of claim 1, wherein the distal end of the first distractor is tapered for wedging between the upper vertebral body and the lower vertebral body.
 3. The system of claim 1, wherein the distal end of the first distractor is bullet-shaped.
 4. The system of claim 1, wherein the distal end of the first distractor is substantially the same shape as the insertion end of the distraction maintenance device.
 5. The system of claim 1, wherein the distal end of the second distractor is tapered for wedging between the upper vertebral body and the lower vertebral body.
 6. The system of claim 1, wherein the distal end of the second distractor is bullet-shaped.
 7. The system of claim 1, wherein the distal end of the second distractor is substantially the same shape as the insertion end of the distraction maintenance device.
 8. The system of claim 1, wherein the first thickness and the second thickness are diameters.
 9. The system of claim 1, wherein the distraction maintenance device comprises surface irregularities on at least one of the upper surface and the lower surface.
 10. The system of claim 9, wherein the surface irregularities include at least one of ratchetings and knurling.
 11. The system of claim 9, wherein the surface irregularities are oriented to resist proximal migration of the distraction maintenance device more than the surface irregularities resist distal migration of the distraction maintenance device.
 12. The system of claim 1, wherein the distraction maintenance device comprises opposite sidewalls between the upper surface and the lower surface.
 13. The system of claim 12, wherein the opposite sidewalls are substantially planar.
 14. The system of claim 1, wherein the distraction maintenance device has a length and a mid-longitudinal axis, the distraction maintenance device having a generally rectangular cross section transverse to the mid-longitudinal axis along at least a portion of the length of the distraction maintenance device.
 15. The system of claim 1, further comprising an inserter adapted to cooperatively engage the distraction maintenance device.
 16. The system of claim 1, wherein the trailing end of the distraction maintenance device is configured to cooperatively engage an inserter for inserting the distraction maintenance device between the upper vertebral body and the lower vertebral body.
 17. The system of claim 1, further comprising a guard for providing protected access to the upper vertebral body and the lower vertebral body, the guard having a passage configured to permit the distraction maintenance device to be inserted therethrough.
 18. The system of claim 1, further comprising a tissue removal device for removing at least a portion of a spinal disc between the upper vertebral body and the lower veterbral body.
 19. The system of claim 18, wherein the tissue removal device is a rotating cutter.
 20. The system of claim 1, further comprising a distractor puller for removing the distractors from between the upper vertebral body and the lower body.
 21. A distraction maintenance device for insertion between an upper vertebral body and a lower vertebral body adjacent a disc space of a human spine, the device comprising: an upper surface for contacting an endplate of the upper vertebral body; a lower surface for contacting an endplate of the lower vertebral body; a trailing end configured to cooperatively engage an inserter; and an insertion end being at least in part curvilinear and adapted to facilitate introduction into the disc space to distract the upper vertebral body away from the lower vertebral body; at least one of the upper surface and the lower surface includes bone engaging projections having a distal sloped surface and a proximal sloped surface, a plane containing at least a portion of the distal sloped surface and a plane containing at least a portion of the proximal sloped surface intersect.
 22. The distraction maintenance device of claim 21, wherein the curvilinear insertion end includes a convex arc between the upper surface and the lower surface.
 23. The distraction maintenance device of claim 21, wherein the curvilinear insertion end is bullet-shaped.
 24. The distraction maintenance device of claim 21, further comprising opposite sidewalls between the upper surface and the lower surface.
 25. The distraction maintenance device of claim 24, wherein the opposite sidewalls are substantially planar.
 26. The distraction maintenance device of claim 21, wherein the bone engaging projections are ratchetings.
 27. The distraction maintenance device of claim 21, wherein the bone engaging projections are oriented to resist proximal migration of the distraction maintenance device more than the bone engaging projections resist distal migration of the distraction maintenance device.
 28. The distraction maintenance device of claim 21, wherein the distal sloped surface is longer than the proximal sloped surface.
 29. The distraction maintenance device of claim 21, wherein projections of the distal sloped surface and the proximal sloped surface meet at an obtuse angle.
 30. The distraction maintenance device of claim 21, wherein projections of the distal sloped surface and the proximal sloped surface meet at an acute angle.
 31. The distraction maintenance device of claim 21, wherein the distraction maintenance device has a length and a mid-longitudinal axis along the length, the distraction maintenance device having a generally rectangular cross section transverse to the mid-longitudinal axis along at least a portion of the length of the distraction maintenance device.
 32. The distraction maintenance device of claim 21, wherein the trailing end of the distraction maintenance device includes a recess adapted to receive at least a portion of the inserter for inserting the distraction maintenance device between the upper vertebral body and the lower vertebral body.
 33. A method of distracting an upper vertebral body from a lower vertebral body adjacent a disc space of a human spine, the method comprising the steps of: inserting a first distractor having a first thickness in contact with at least a portion of the endplates of the upper vertebral body and the lower vertebral body; inserting a second distractor having a second thickness that is greater than the first thickness in contact with at least a portion of the endplates of the upper vertebral body and the lower vertebral body; and inserting between the upper vertebral body and the lower vertebral body a distraction maintenance device having an upper surface, a lower surface, an insertion end, and a trailing end.
 34. The method of distracting of claim 33, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with opposite sidewalls between the upper surface and the lower surface.
 35. The method of distracting of claim 34, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with substantially planar opposite sidewalls.
 36. The method of distracting of claim 33, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with bone engaging projections on at least one of the upper surface and the lower surface.
 37. The method of distracting of claim 36, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with bone engaging projections that are ratchetings.
 38. The method of distracting of claim 36, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with bone engaging projections that are oriented to resist proximal migration of the distraction maintenance device more than the bone engaging projections resist distal migration of the distraction maintenance device.
 39. The method of claim 33, wherein the method is performed as a bilateral procedure.
 40. The method of claim 33, wherein at least one of the steps of inserting the first and second distractors and the distraction maintenance device are performed through a guard.
 41. The method of claim 33, further comprising the step of inserting a fusion promoting material between the upper and lower vertebral bodies.
 42. A method of distracting an upper vertebral body from a lower vertebral body adjacent a disc space in the human spine, the lower vertebral body being connected to the upper vertebral body by one or more tissue structures, the method comprising the steps of: sequentially inserting and removing at least two distractors of progressively greater thicknesses into the disc space and in contact with at least a portion of the endplates of the upper vertebral body and the lower vertebral body until at least one of the following sub-steps occurs: (a) a normal height between the upper vertebral body and the lower vertebral body is achieved, (b) the tissue structures between the upper vertebral body and the lower vertebral body become tensioned to increase stability between the upper vertebral body and the lower vertebral body, and (c) a surgeon determines that the stability between the upper vertebral body and the lower vertebral body adequately resists further distraction; and inserting between the upper vertebral body and the lower vertebral body a distraction maintenance device with an upper surface, a lower surface, an insertion end, and a trailing end.
 43. The method of distracting of claim 42, wherein the step of sequentially inserting and removing at least two distractors includes inserting and removing distractors of progressively greater diameters.
 44. The method of distracting of claim 42, wherein the step of inserting a distraction maintenance device includes inserting a device with substantially the same shape as the last distractor sequentially inserted in the step of sequentially inserting and removing at least two distractors.
 45. The method of distracting of claim 42, wherein the step of inserting a distraction maintenance device includes inserting a device with substantially the same insertion end shape as a proximal end of the last distractor sequentially inserted in the step of sequentially inserting and removing at least two distractors.
 46. The method of distracting of claim 42, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with a curvilinear insertion end.
 47. The method of distracting of claim 42, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with a convex arc between the upper surface and the lower surface.
 48. The method of distracting of claim 42, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with a bullet-shaped insertion end.
 49. The method of distracting of claim 42, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with opposite sidewalls between the upper surface and the lower surface.
 50. The method of distracting of claim 49, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with substantially planar opposite sidewalls.
 51. The method of distracting of claim 42, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with bone engaging projections on at least one of the upper surface and the lower surface.
 52. The method of distracting of claim 51, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with bone engaging projections that are ratchetings.
 53. The method of distracting of claim 51, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with bone engaging projections that are oriented to resist proximal migration of the distraction maintenance device more than the bone engaging projections resist distal migration of the distraction maintenance device.
 54. The method of claim 42, wherein the method is performed as a bilateral procedure.
 55. The method of claim 42, wherein at least one of the steps of inserting the first and second distractors and the distraction maintenance device are performed through a guard.
 56. The method of claim 42, further comprising the step of inserting a fusion promoting material between the upper and lower vertebral bodies.
 57. A method of determining an effective distraction of an upper vertebral body from a lower vertebral body adjacent a disc space in a human spine, the lower vertebral body being connected to the upper vertebral body by one or more tissue structures, the method comprising the steps of: inserting a first distractor having a first thickness into the disc space and in contact with at least a portion of the endplates of the upper vertebral body and the lower vertebral body, the first thickness being sufficient to move the upper vertebral body and the lower vertebral body apart; determining if the fit of the first distractor is sufficiently tight in the disc space to achieve a desired amount of distraction of the upper vertebral body and the lower vertebral body, and performing one of the following additional steps: (a) inserting a distraction maintenance device between the upper vertebral body and the lower vertebral body if the fit of the first distractor is sufficiently tight in the disc space to achieve a desired amount of distraction of the upper vertebral body and the lower vertebral body; or (b) sequentially inserting additional distractors of thicknesses that are sequentially greater than the first thickness between the upper vertebral body and the lower vertebral body if the fit of the first distractor is not sufficiently tight in the disc space until the fit of a distractor is sufficiently tight in the disc space to achieve the desired amount of distraction of the upper vertebral body and the lower vertebral body, and then inserting a distraction maintenance device between the upper vertebral body and the lower vertebral body.
 58. The method of claim 57, wherein the step of inserting the first distractor includes impacting the first distractor to move the distractor inbetween the upper vertebral body and the lower vertebral body.
 59. The method of claim 57, wherein the step of determining if the fit of the first distractor is sufficiently tight includes at least one of pushing, pulling, sliding, and applying torque to the first distractor.
 60. The method of claim 57, wherein the step of determining if the fit of the first distractor is sufficiently tight includes assessing forces applied to the tissue structures.
 61. The method of claim 57, wherein the step of sequentially inserting distractors includes impacting the distractors to move the distractors inbetween the upper vertebral body and the lower vertebral body.
 62. The method of claim 57, wherein the step of inserting the distraction maintenance device includes impacting the distraction maintenance device to move the distraction maintenance device inbetween the upper vertebral body and the lower vertebral body.
 63. The method of claim 57, wherein the step of inserting a distraction maintenance device includes inserting the distraction maintenance device having substantially the same shape as the last distractor inserted.
 64. The method of claim 57, wherein the step of inserting a distraction maintenance device includes inserting the distraction maintenance device having substantially the same insertion end shape as a proximal end of the last distractor sequentially inserted.
 65. The method of claim 57, wherein the step of inserting the first distractor is performed through a guard.
 66. The method of claim 57, wherein the method is performed as a bilateral procedure.
 67. The method of claim 57, further comprising the step of inserting a fusion promoting material between the upper and lower vertebral bodies.
 68. A system for spacing apart an upper vertebral body and a lower vertebral body adjacent a disc space in a human spine, the system comprising: a first distractor having a proximal end, a distal end, and a first thickness adapted to separate the upper vertebral body from the lower vertebral body along the first thickness, the first distractor being configured to contact at least a portion of the upper vertebral body and the lower vertebral body; a second distractor having a proximal end, a distal end, and a second thickness that is greater than the first thickness, the second distractor adapted to separate the upper vertebral body from the lower vertebral body along the second thickness, the second distractor being configured to contact at least a portion of the upper vertebral body and the lower vertebral body; and a distraction maintenance device having an upper surface, a lower surface, an insertion end, and a trailing end, the distraction maintenance device having a height adapted to maintain a space between the upper vertebral body and the lower vertebral body, the insertion end having a rounded portion from the upper surface to the lower surface of the distraction maintenance device.
 69. The system of claim 68, wherein the distal end of the first distractor is tapered for wedging between the upper vertebral body and the lower vertebral body.
 70. The system of claim 68, wherein the distal end of the first distractor is bullet-shaped.
 71. The system of claim 68, wherein the distal end of the first distractor is substantially the same shape as the insertion end of the distraction maintenance device.
 72. The system of claim 68, wherein the first thickness and the second thickness are diameters.
 73. The system of claim 68, wherein the distraction maintenance device comprises surface irregularities on at least one of the upper surface and the lower surface.
 74. The system of claim 73, wherein the surface irregularities include at least one of ratchetings and knurling.
 75. The system of claim 73, wherein the surface irregularities are oriented to resist proximal migration of the distraction maintenance device more than the surface irregularities resist distal migration of the distraction maintenance device.
 76. The system of claim 68, wherein the distraction maintenance device comprises opposite sidewalls between the upper surface and the lower surface.
 77. The system of claim 76, wherein the opposite sidewalls are substantially planar.
 78. The system of claim 68, wherein the distraction maintenance device has a length and a mid-longitudinal axis, the distraction maintenance device having a generally rectangular cross section transverse to the mid-longitudinal axis along at least a portion of the length of the distraction maintenance device.
 79. The system of claim 68, further comprising an inserter adapted to cooperatively engage the distraction maintenance device.
 80. The system of claim 68, wherein the trailing end of the distraction maintenance device is configured to cooperatively engage an inserter for inserting the distraction maintenance device between the upper vertebral body and the lower vertebral body.
 81. The system of claim 68, further comprising a guard for providing protected access to the upper vertebral body and the lower vertebral body, the guard having a passage configured to permit the distraction maintenance device to be inserted therethrough.
 82. The system of claim 68, further comprising a tissue removal device for removing at least a portion of a spinal disc between the upper vertebral body and the lower veterbral body.
 83. The system of claim 82, wherein the tissue removal device is a rotating cutter.
 84. The system of claim 68, further comprising a distractor puller for removing the distractors from between the upper vertebral body and the lower body.
 85. A method of distracting an upper vertebral body from a lower vertebral body adjacent a disc space of a human spine, the method comprising the steps of: inserting a first distractor having a first thickness in contact with at least a portion of the upper vertebral body and the lower vertebral body; inserting a second distractor having a second thickness that is greater than the first thickness in contact with at least a portion of the upper vertebral body and the lower vertebral body; and inserting between the upper vertebral body and the lower vertebral body a distraction maintenance device having an upper surface, a lower surface, an insertion end, and a trailing end, the insertion end having a rounded portion from the upper surface to the lower surface of the distraction maintenance device.
 86. The method of claim 85, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with opposite sidewalls between the upper surface and the lower surface.
 87. The method of claim 86, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with substantially planar opposite sidewalls.
 88. The method of claim 85, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with bone engaging projections on at least one of the upper surface and the lower surface.
 89. The method of claim 88, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with bone engaging projections that are ratchetings.
 90. The method of claim 88, wherein the step of inserting a distraction maintenance device includes inserting a distraction maintenance device with bone engaging projections that are oriented to resist proximal migration of the distraction maintenance device more than the bone engaging projections resist distal migration of the distraction maintenance device.
 91. The method of claim 85, wherein the method is performed as a bilateral procedure.
 92. The method of claim 85, wherein the steps of inserting the first and second distractors are performed through a guard.
 93. The method of claim 85, further comprising the step of inserting a fusion promoting material between the upper and lower vertebral bodies. 